For safety and economic reasons, aircraft must have the capability of determining accurately the amount of fuel on board. Federal regulations determine the minimum amount of fuel which an aircraft must carry because unexpected events may alter the course and destination of the aircraft. In these situations it is important to have an accurate measurement of fuel remaining to enable the craft to be flown safely to an alternate destination.
The amount of fuel to be carried by an aircraft is determined by a variety of considerations, such as payload, route, weather, etc. The heavier the fuel load, the less payload an aircraft can carry. Therefore, accurate on-board fuel gauging equipment is commercially desirable because it allows less fuel to be carried for a given route, resulting in a more economic flight operation.
Conventional fuel gauging systems have used capacitance probes located in the tanks of aircraft. Generally, such probes consist of two concentric vertically-disposed electrodes defining a space in which fuel rises during filling and lowers during consumption. The level of the fuel within the probe alters the capacitance value of the probe. The capacitance of the probe varies linearly with the fuel level within the probe. Therefore, an electrical signal applied to the probe can be used to determine a probe capacitance value at a given fuel level. This capacitance value can then be used to determine the quantity of fuel in the tank.
One such commercially available fuel system is known as the Summed Characterized Probe System. In this system, each probe has a longitudinal capacitance characteristic tailored to match the specific fuel tank characteristics at each fuel level. The diameter of the inner electrode of the probe is altered longitudinally by various processes, such as swaging. All of the probes located in the tank are then electronically summed to provide a signal that is proportional to total fuel quantity in the tank. This system has the advantage of requiring minimal wiring, and the required electronics are relatively simple. However, a drawback of the system is that the probes are expensive to manufacture, and upon probe failure, the entire system can become inoperable. With this system, it is very difficult to achieve fuel quantity determinations with a high degree of accuracy and still maintain a robust mechanical design.
A second type of commercially available fuel gauging system is known as the Individually Addressed Linear Probe System. In this type system, the capacitance of each probe is measured individually. Each probe is linear, i.e. the diameters of inner and outer electrodes remain constant throughout the length of the probe. Therefore, the probes are relatively inexpensive to manufacture. The integrity of each individual probe is monitored to detect probe failure; however, with such a system, there is an increased complexity of the monitoring electronics and wiring. The additional wiring adds to the weight and cost of the system, and reduces system reliability. The electronics are more complicated because each probe must be driven separately in order to achieve a reasonable system response to fuel level changes. This added complexity makes such a system expensive, and it also reduces system reliability.
One example of a prior art capacitative fuel gauging system is disclosed in U.S. Pat. No. 4,918,619. The '619 patent suggests using a junction probe to multiplex the outputs from several probes and transmit the results in digital format to appropriate readout displays. The disadvantage of this system is that there are active electronic components in the tank. In addition, a substantial amount of electronics is required, and this can lead to reliability problems.
Prior art fuel gauging systems all rely on the same basic principle of capacitance measurement. As discussed, these represent a trade-off between performance and cost. Although various fuel gauging systems of the prior art may function satisfactorily for their intended purposes, there is a need for a more accurate fuel gauging system which meets performance objectives while minimizing weight and cost.